In general, filtration involves the removal of suspended solids and/or dissolved impurities from a fluid by passing the fluid through a porous medium. For many applications, the porous medium traditionally has been a bed of particulate matter, such as activated carbon and diatomaceous earth, or a solid porous filter element composed primarily of activated carbon. Solid porous filter elements are especially desirable for ease of handling and ready disposability. They may be made by a batch process, although it is both time consuming and labor intensive. More recently, they may be formed continuously by extruding a mixture of a thermoplastic binder material and a powdered or granular primary material such as activated carbon.
Apertured films, woven fabrics, and nonwoven materials also have been used as filter materials for removing or separating particles from liquids. Generally speaking, such filter materials rely on some form of mechanical straining or physical entrapment. Such filter sheets can pose limitations when the size of the particle to be removed is small relative to the average pore diameter of the filter sheet. For nonwoven materials, this is particularly true for particles of less than one micrometer in diameter.
Improved filters have been developed with modified surface charge characteristics to capture and adsorb particles by electrokinetic interaction between the filter surface and particles contained in an aqueous liquid. Such charge-modified filters typically consist of microporous membranes or involve the use of materials which are blends of glass fibers and cellulose fibers or blends of cellulose fibers and siliceous particles. Such filters may also utilize packed beds of particles. Charge modification generally is accomplished by coating the membrane or at least some of the fibers with a charge-modifying agent and then treating with a separate crosslinking agent in order to ensure the durability of the coating.
While microporous membranes generally are capable of effective filtration, flow rates through the membranes typically are lower than for fibrous filters. Moreover, microporous membranes generally have higher back pressures during the filtration process than do fibrous filters.
With the exception of reverse osmosis and, to a limited extent, hollow fiber membranes, no current filtration technology can effectively remove harmful bacteria and viruses from water. Usually, chemical and ultraviolet disinfection are the only reliable methods for eliminating biological contamination.
Typical activated carbon filter elements, such as those described earlier, are very efficient at removing most aesthetic contaminants from water, e.g., sediment, residual chlorine, and other contaminants which affect taste and odor. Such filter elements also are capable of removing certain health-threatening substances, such as lead and volatile organic compounds. Moreover, recent improvements in the manufacture of block activated carbon elements now permit the efficient removal of certain biological contaminants of larger sizes, such as Cryptosporidium and Giardia. However, in almost all cases the smaller bacteria can readily pass through these filter elements, and those which are trapped then are able to reproduce within the filter element. The continuous or frequent filtration of biologically contaminated water can cause the bacterial levels within the filter to reach several orders of magnitude higher than the levels in the water being filtered. Unless care is taken, this bacterial overgrowth can be flushed directly into the water to be consumed.
Accordingly, there is a need for a filter which has the ability to remove all or significantly reduce contaminants from water, including sediment, organic compounds, odors, chlorine, lead, and bacterial and viral pathogens.